A mechanism by which a host defends against trauma involves leukocytic infiltration followed by phagocytosis. In the infiltration process, leukocytes present in circulating blood stick to blood vessel walls, emigrate through the wall (diapedesis), and migrate to the site of the trauma. The migration may be random or directional. Directional migration is termed "chemotaxis". In the phagocytotic (cellular eating) process, leukocytes attach to foreign matter at the traumatic site, engulf and injest it and, finally, kill and digest the offensive bodies. In so doing, leukocytes destroy matter associated with the trauma, cleanse the traumatic site, and provide future protection against trauma. Agents which increase the motility of leukocytes from the circulation to the site of the traumatic attack would thus enhance the host's ability to defend against such attack. We have now have found that xanthines promote leukocytic locomotion, thereby enhancing the host's defense mechanisms.